Wave-guide isolation coupling system



w. c. BABCOCK ETAL 2,639,371

WAVE-GUIDE ISOLATION COUPLING SYSTEM May 19, 1953 2 Sheets-Sheet 1 Filed Jan. 25, 1950 PARABOLIC DISH AN 7:

BPOADCAS T TRANSMI 7' TER WC. BABCOCK H. WNVLUND INVENTORS AGENT Patented May 19, 1953 UNITED, STATES PATENT OFFICE WAVE-GUIDE ISOLATION COUPLING SYSTEM Wallace 0. Babcock, Bayside, and Harry W. Nylund, Great Neck, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 25, 1950, Serial No. 140,434

4 Claims.

This invention relates to antenna systems'for electromagnetic waves whose frequencies lie in different portions of the frequency spectrum.'

In point-to-point microwave transmission systems, it'is the practice, because of the limitation to substantially line-of-sight transmission, to mount'the microwave antennae 'at a considerableheight above groundindividual towers being erected for'this purpose when necessary.

Throughout the country, there are mastor tower-type antennae used for radio broadcasting, and often it would be advantageous if these could be used to provide the necessary elevated support for microwave antennae. Many of these tower-type antennae, however, have insulated bases and are operated with a high radio-frequency potential to ground. A hollow-pipe wave guide extending dow'nthe tower from a microwaveantenna to a microwave transmitter or receiver on the'ground would, in such case, shortcircuit the tower inasmuch as the radio-frequency power would flow over the exterior of the wave guide to ground.

An object of the presentinventionis to enable such an insulated tower-type broadcast antenna to be used as 'a support for a microwave antenna and its associated wave-guide transmission line.

A related object is to obviate the necessity for high voltage insulation of the wave guide from the tower and to enable the wave guide to be conductively secured throughout its length to the tower.

A feature of the' invention is a microwave isolation coupler interposed in the wave guide near the base of the tower, the coupler providing a substantially continuous low-pass path for the transmission of microwaves to or from the micro wave antenna and introducing in the external circuit provided by the exterior of the wave guide an open circuit or gap that cannot be bridged by the high power, broadcast frequency currents.

The nature of the present invention will appear more fully in consideration of the following de scription of a preferred embodiment of the invention, reference being made also to the appended drawings, in which:

Fig. 1 shows an embodiment of the invention in which an insulated broadcast tower-type antenna supports a microwave antenna and hollow wave-guide feeder therefor;

Fig. 2 shows in partial section a wave-guide isolation coupler in accordance'with the invention;

Fig. 3 shows the wave-guide isolation coupler with-attached neoprene cover; 'ande 2 Figs. 4 and 5 are curve diagrams showing the characteristics of the isolation coupler.

- Referring t Fig. 1, designates an insulated tower antenna erected to a suitable elevation, for example, 400 to 500 feet above the ground. The antenna tower I, which is insulated from the ground by an insulator 2 and concrete base 3, is used for radio broadcasting and is fed from a radio-frequency high power transmitter by a coaxial transmission line and transformer coupling, as disclosed by the United States Patent 2,140,174, issued December 13, 1938, to P. H. Smith. In one exemplary installation, the radio broadcast frequency was 1150 kilocycles. I

An antenna 5 for microwaves, namely, a. parab oloidal dish, or the like, is supported on a horizontal platform connected to tower antenna approximately half-way above the ground. The paraboloidal antenna 5, which may be of the type disclosed in the United States Patent No. 2,422,184, issued June 1'7, 1947, to C. C; Cutler, is connected to a feeder line comprising a vertical section of wave guide 6 (1.5 x 3.0 inches in cross section), a -degree wave guide bend 1, an isolation coupler-8, and a horizontal section of wave guide l0 connected to either a microwave transmitter, or a receiver. The latter may be of the typedisclosed in anarticle by J. F. Wentz and K. D. Smith entitled A new microwave television system in the Trans. A. I. E. E., volume 66, pages 465 through 4'70, 194-7, adapted for microwave ra-" dio relay use. The vertical section of wave guide 6 is fastened to the tower by bolts or other fastening means, while the horizontal section is supported at an elevation of nine feet from the ground.

Fig. 2 shows the isolation coupler in full detail excepting for the cover.

The coupler 8 comprises two hollow rectangular wave-guide sections 2|, 22 separated by a three-foot gap, which is bridged by a rectangular polystyrene bar 23 about 40 inches in length, extending a few inches int the proximate ends of each'wave guide 2|, 22. These proximate ends of wave guides 2|, 22 are terminated by pyramidal horns 24, 25 facing each other for beaming microwaves therebetween along the surface of and into the dielectric bar 23. The horns 24, 25are spaced apart mechanically and electrically insulated from each other by four rods 26, formed of phenol fibre.

The length of polystyrene bar 23, or, more parl to ground and to preclude the introduction of excessively low reactance paths to ground at the radio broadcasting frequencies.

A cylindrical protective cover 28 of neoprene is clamped to the circular grooved edges 29 of the horns to act. as a gas-tight seal; for the inert gas flowing through. thewave-gui'de feeder and to prevent rain and dust from infiltrating thereinto, as illustrated in Fig. 3.

The polystyrene bar 23 is provided with tapered terminals 30, for matching the impedance thereof to that of the wave guides 2|, 22. The corners of the bar, where the tapered ends protrude into the wave guides, are; slightly chamfered at 32 for permitting the flowof nitrogenv or other inert gas under pressure into. the'waveguide system. The cross-sectional.dimensions of the bar 23 permit its ends to fit snugly into wave guides 2 l, 22.

A dielectric bar 23 connecting two open-ended wave guides 2 t, 22'. wild concentrate and transmit microwave energy between the guides and across the. gap... In the transmission through the bridgpolystyrene bar, however, some of. the energy may tend to travel along the exterior surface of the. bar and may tend to; escape into space: or be reflected when it encounters; the impedance discontinuity prcseirt where the wave guide. ends, namely at. 2t, 22".. The pyramidal horns 24:, 25, which terminate the wave guides 21', 22, cap-- ture this. energy direct. it back into wave uide: sections and also overcome. to a. considerable extent the aforementioned impedance discontinuity.

The insertion loss: of the polystyrene isolation coupler s is low, ranging from. 0.3 to. 0.5 decibel in the. microwave band from 3.75 to 4.25 kilomegacyc'l'es. Low loss is; an essential characteristic. and low dielectric constant is: a desirable characteristic of the isolation coupler, particularly for simplifying the impedance-matching problem;

As shown in Fig. 4, the voltage standing Wave ratio over this microwave band ranges from a minimum of 0.1 decibel to amaximum. of 3-.2 decibels in cyclic fashion, adjacent minima being about lid megacyc-les apart. This cyclic variati'orr of voltage standing wave ratio with frequency depicted in Fig. 4 is attributable to the vector addition of reflections originating at each end oi'the isolation coupler and caused by:

1:. junction between wave guide and horn;

2. The transition from the end points of the polystyrene bar into the hollow guide space;

3'. Aperture ofthe horn.

The isolation coupler is suitable for broadoperation over a. frequency band 0.4 to .5 kilomegacycle wide centered around" 45 kilomega cycles, where a standing wave ratio of about 2 to 23 decibels is permissible.

The; exact frequencies at which minimum standing wave ratios are found are critically dependent upon the dimensions of the parts of the coupler. With proper dimensions of horns and polystyrene bar, it is possible to obtain a V. S. W. R. minimum of 0.2 decibel at 3970 megacycles. A typical set of dimensions utilized for achieving this result comprised a bar 39% inches long, 1 inches wide, tapered over a length of 1% inches at its ends. The height of the horn was 5 inches, and the rectangular mouth thereof was 6 inches by 5% inches, while the horns circular-base has a diameter oi5% inches.

At any desired frequency, one may choose horn 4 spacings corresponding to a minimum S. W. R., as shown in Fig. 5.

It should be understood that other dielectrics, which sow low loss in the microwave range, may be used in the isolation coupler, for example, quartz; methylmethacry-latc resin, glass, mica bonded glass, or the like.

It should also be understood that for narrowband operation, dielectric bars of low loss and high dielectric constant may be utilized, such as rutile, barium titanate, strontium titanate, and the like.

It should "also be understood that the paraboloidal: microwave antenna shown is merely exempi ary and that other types, such as microwave horns, slotted waveguides, arrays of dipoles, metallic lens antennae, or the like, may be substituted therefor without departing from the spirit of' the invention.

The polystyrene bar may be cylindrical, hexagonsd, octagonal, or the like in shape, and it may be reduced. in cross section. without departing from the spirit of. the invention.

What is-claimed is 1 l. In; combination, a hollow-pip wave guide pressurized by an inert gas and transmitting therethrou h microwaves, said guide having a gap intermediate its ends, a low-loss dielectric bar bridging said gap, said bar having impedance-matchi-ng gas-to-d-ielectric transition sections' of substantially reduced cross-sectional dimensinos at each end thereofi, the portions of said bar extending into said pressurized wave guides being of slightly reduced peripheral dimensions relative to the enclosing wav guides to permit passage at gas around the ends of said bar, opposed inwardly extendingv conductive horns secured to the ends of said wave guide on either side.- of said gap to concentrate the microwaves and: supplement said dielectric bar in their 7 base, an elevated microwave antenna supported on tower, microwave equipment spaced from said tower, a conductive hollow pipe wave guide extending between said microwave antenna and equipment for the propagation of microwaves therebetween, said wave guide being conductivel-y secured to said antenna tower substantially throughoutx its length between said base and said m1crowave antenna, said wave guide having a non-conductive gap therein adjacent said base, and a low-loss dielectric bar extending across said gap and into the ends of the wave guide on both. sides: of said gap to bridge said gap for the transmission. of microwaves and the prevention of flashover from the high. voltage antenna tower along said wave guiderto ground.

3. In combination, a high voltage radio frequency broadcasting tower, a broadcast transmitter coupled to said tower, insulating means supporting; the: base of said tower at predetermined height above ground, microwave equipment, spaced from said tower, an. electrical conduotor connecting said. microwave equipment to ground, an elevated microwave antenna mounted on said tower, a.- oonducti-ve hollow pipe wave guide extending between said. microwave antenha and equipment for the propagation of micro waves therebetween, said wave guide being conductively secured to said tower substantially throughout its length between said base and said microwave antenna, said wave guide having a non-conductive gap therein at a location between said microwave equipment and the base of said tower, and a low-loss dielectric bar extending across said gap to the ends of the wave guide on both sides of said gap to bridge said gap for the transmission of microwaves, the length of said non-conductive gap and the distance of said wave guide above ground between said gap and the tower base both being greater than said predetermined height, whereby fiashover from said high voltage tower to ground through said wave guide or said gap is prevented.

4. In combination, a high power radio frequency transmitter, a radiating antenna tower coupled thereto and insulated from ground at its base, an elevated microwave antenna supported on said tower, microwave equipment spaced from said tower, a pressurized conductive hollow pipe wave guide extending between said microwave antenna and equipment for the propagation of microwaves therebetween, said wave guide being conductively secured to said antenna tower substantially throughout its length between said base and said microwave antenna, said Wave guide having a non-conductive gap therein adjacent said base, a low-loss dielectric bar extending across said gap and into the ends of the wave guide on both sides of said gap to bridge said gap for the transmission of microwaves and the prevention of fiashover from the high voltage antenna tower through the microwave equipment to ground, the portions of said dielectric bar which extend into said wave guides being slightly reduced in cross section relative to the enclosing wave guides, to permit the passage of pressurized gas, opposed inwardly extending conductive horns secured to the ends of said wave guide on either side of said gap to concentrate the microwaves and supplement said dielectric bar in its transmission of microwaves from one section of conductive wave guide to the other, and a gastight cover of non-conductive material secured to the outer periphery of said horns and sealing said gap therewithin to facilitate the transmission of the pressurized gas across the WALLACE C. BABCOCK. HARRY W. NYLUND.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,912,794 Peterson June 6, 1933 2,155,508 Schelkunofi Apr. 25, 1939 2,158,875 Leeds May 6, 1939 2,313,850 Usselman Mar. 16, 1943 2,393,218 Caraway Jan. 15, 1946 2,427,098 Keizer Sept. 9, 1947 2,463,297 Muskat et al Mar. 1, 1949 2,503,549 Fox Apr. 11, 1950 2,535,251 Alford Dec. 26, 1950 FOREIGN PATENTS Number Country Date 881,161 France Apr. 16, 1943 

